UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS 

IN 

AGRICULTURAL    SCIENCES 

Vol.  1,  No.  2,  pp.  21-37  October  15,  1912 


STUDIES   ON   THE   PHENOLDISULPHONIC 

ACID  METHOD  FOR  DETERMINING 

NITRATES  IN  SOILS 


BY 

C.  B.  LIPMAN  and  L.  T.  SHARP 


Despite  the  fact  that  some  careful  research  has  been  carried 
out  on  the  colorimetric  method  for  determining  nitrates,  many 
factors  concerned  with  it  have  not  been  studied,  and  the  some- 
what uncertain  nature  of  the  method  makes  it  imperative  to  con- 
trol, so  far  as  possible,  every  factor  which  may  interfere  with 
the  accurate  analysis  of  nitrate-containing  material.  These 
statements  apply  particularly  to  the  analysis  of  soils  for  nitrates 
and  the  authors  therefore  deem  the  subjoined  data,  derived  from 
a  thorough  investigation,  deserving  of  the  attention  of  every  soil 
chemist. 

Among  the  interfering  factors  in  the  phenoldisulphonic  acid 
method  which  have  been  either  studied  inadequately  or  not  at 
all,  are  the  effects  of  salts,  the  effects  of  agents  employed  to  pre- 
cipitate the  clay  and  organic  matter,  and  the  effects  of  decolor- 
izing agents.  Cognizance  must  be  taken  of  all  of  these  factors 
by  the  chemist  in  the  determination  of  nitrates  and  in  the  ar- 
rangement and  interpretation  of  results.  The  importance  of  salt 
effects  and  their  significance  in  this  connection  are  emphasized 
by  the  fact  that  many  soils,  and  particularly  those  of  arid  and 
semiarid  regions,  may  frequently  be  found  to  a  greater  or  less 
degree  impregnated  with  one  or  more  of  the  so-called  "alkali 
salts,"  together  with  which,  it  often  happens  indeed,  consider- 


22  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 

able  quantities  of  nitrates  are  to  be  found.     So  far  as  the  clay- 
coagulating  substances  are  concerned,  it  has  always  been  a  com-  A 
mon  practice  in  soil  work  to  employ  varying  amounts  of  a  satur- 
ated solution  of  alum  to  obtain  a  clear  soil  solution,  and  more 
recently  it  has  been  proposed  by  investigators  who  have  studied 
the  method  under  discussion  to  use  aluminum  cream  for  the  pur-  > 
pose  in  place  of  alum.    For  decolorizing  solutions  both  aluminum 
cream  and  bone  black  have  been  used.     The  methods  for  both 
clay  coagulation  and  decolorization  are  obviously  essential  in  most                               * 
soil   work,   since   ordinary   filtration,    without   the   use   of   such 
agents,  can  rarely  be  depended  on  to  yield  a  clear,  colorless  soil 
solution,   even  if  time  be  no  object.     The  employment   of  the 
Pasteur  Chamberland  filter  to  remove  clay  has  been  found  by                                a 
direct  investigation  to  involve  well-defined  losses  of  nitrates. 

It  is  not  our  purpose  here  to  enter  into  a  lengthy  review  of 
other  investigations  bearing  on  the  subject  in  hand,  but  into  a 
brief  discussion  of  the  more  important  ones  which  show  the  ques-  ^ 

tions  still  remaining  unsolved  or  bring  out  certain  results  with 
which  ours  do  not  agree. 

In  1894  Gill1  carried  out  a  series  of  painstaking  investiga-  * 

tions  which,  briefly,  indicate  (1)  that  for  purposes  of  accuracy 
the  phenoldisulphonic  acid  employed  in  the  nitrate  determination 
must  be  carefully  prepared  to  insure  a  uniform  compound  for 
use  as  a  standard;  (2)  that  chlorine  induces  losses  of  nitric  acid  « 

both  when  the  solution  containing  nitrate  is  evaporated  on  the 
water  bath  and  when  the  residue  is  treated  with  the  reagent; 
(3)  that  Na2C03  added  to  the  nitrate-containing  solution  to  pre-  * 

vent  escape  of  nitric  acid  during  evaporation  induces  losses  of 
nitrates  varying  in  quantity  from  four  to  six  per  cent;  (4)  that 
alumina  may  be  used  to  precipitate  colloidal  material  for  obtain- 
ing a  clear  solution  ;  (5)  that  silver  sulphate,  if  free  from  nitrate,  « 
may  be  employed  to  precipitate  chlorine,  thus  removing  an  im- 
portant interfering  agent. 

More  recently  Chamot  and  his  coworkers2  have  prosecuted  an 
even  more  thoroughgoing  investigation  than  the  preceding,  in 
which  the  most  emphasis  has  been  placed,  however,  on  the  mode 
of  preparation  of  the  tripotassium  salt  of  nitrophenoldisulphonic 


i  Jour.  Am.  Chem.  Soc.  vol.  1G,  p.  122.     1894. 


1912]  Lipman- Sharp :   Phenoldisulphonic  Acid  Method  23 

acid  used  as  the  reagent.  Their  results  indicate  (1)  that  in 
order  to  obtain  the  phenoldisulphonic  acid  free  from  the  mono 
and  tri-phenolsulphonic  acid  a  careful  digestion  of  the  phenol 
and  sulphuric  acid  under  certain  constant  conditions  must  be 
assured;  (2)  that  the  mono  and  tri-phenolsulphonic  acids  intro- 
duce other  colors  which  interfere  with  the  readings  in  the  colori- 
meter; (3)  that  the  tri-potassium  salt  of  nitrophenoldisulphonic 
acid  gives  the  characteristic  color  employed  in  the  determination 
and  should  always  be  used  as  a  standard;  (4)  that  heating  the 
dry  residue  of  nitrates  even  for  several  hours  on  the  water  bath 
occasions  no  losses;  (5)  that  aluminum  cream  is  the  best  pre- 
cipitating agent  for  organic  matter  of  several  used  and  occasions 
no  losses  of  nitrates;  (6)  that  2  c.c.  of  the  phenoldisulphonic  acid 
should  be  used  in  uniform  amounts  in  all  determinations;  (7) 
that  KOH  was  to  be  preferred  to  NaOH  and  NH40H,  as  the 
alkali  employed;  (8)  that  chlorides  induced  losses  of  nitrates; 
(9)  that  carbonates  and  organic  matter  did  likewise;  (10)  that 
temperature,  concentration,  and  length  of  exposure  to  reagent 
greatly  affect  results;  and  (11)  that  there  have  been  other  minor 
effects  of  iron,  magnesium,  and  nitrites. 

Reference  must  also  be  made  here  to  the  brief  investigation  of 
Stewart  and  Greaves3  pertaining  to  the  effect  of  chlorine  in  de- 
termining nitrates  in  soils,  both  because  the  work  is  recent  and 
because  it  is  the  only  one  published  which  is  derived  from  re- 
searches on  soils.  This  investigation  and  those  above  reviewed 
cover  most  completely  the  questions  involved  and  reference  will 
be  made  in  the  discussion  of  our  experimental  work  below  to 
those  questionable  points  which  were  considered  settled  but  which 
our  work  shows  were  far  from  being  so. 

The  Interference  of  Salts  with  the  Nitrate  Determination 

As  has  been  above  indicated  the  salt  accumulations  which 
occur  in  the  soils  of  California,  Nevada,  Utah,  and  other  arid 
or  semi-arid  regions  frequently  contain  considerable  quantities 
of  nitrates  and  the  determination  of  the  latter  in  the  presence 
of  the  "alkali  salts"  is,  as  has  been  found,  frequently  attended 


2  Ibid.,  vols.  21,  p.  922;  32,  p.  630;  33,  p.  366 
z  Ibid.,  vol.  32,  p.  756. 


24 


University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


with  losses  of  nitric  acid.  While  all  the  investigations  above  re- 
viewed have  pointed  out  the  interference  of  chlorine  and  chlorides 
with  the  nitrate  determination,  and  while  some  of  them  have 
also  considered  the  losses  occurring  through  the  use  of  Na2C03, 
no  mention  is  made  of  the  effects  of  the  most  common  and  widely 
spread  of  the  alkali  salts,  Na2S04,  or  Glauber  salt.  It  seems  fur- 
ther to  have  been  taken  for  granted  that  Na2C03  and  Na2S04 
should,  for  obvious  reasons,  have  the  same  effects  on  the  nitrate 
determination  by  the  phenoldisulphonic  acid  method.  Our  re- 
sults do  not,  however,  bear  out  this  opinion.  Under  this  head 
were  also  studied  the  effects  of  the  kation  as  well  as  the  anion 
of  salts  on  the  same  determination. 

Varying  quantities  of  the  salts  tested  were  here  added  to  the 
same  amounts  of  nitrates  in  solution,  and  uniform  quantities  of 
salts  were  also  tested  as  to  their  effects  on  varying  quantities 
of  nitrates.  Everyone  of  the  following  tables  gives  the  effects 
of  one  of  the  salts  tested  in  accordance  with  the  scheme  above 
indicated  and  in  some  cases  also  shows  how  the  nitrate  deter- 
mination is  affected  by  varying  the  quantities  of  both  the  nitrates 
and  other  salts.  The  residue  containing  the  salts  and  the  nitrates 
was  treated  with  2  c.c  of  phenoldisulphonic  acid  thoroughly 
stirred  for  about  two  or  three  minutes,  25  c.c.  of  nitrate-free  dis- 
tilled water  was  added,  and  then  strong  ammonia  drop  by  drop 
until  the  odor  of  ammonia  persisted  and  the  color  was  per- 
manent. The  solution  was  then  diluted  as  necessary  and  com- 
pared in  the  Sargent-Kennicott  colorimeter  with  a  standard 
solution  similarly  and  always  freshly  prepared,  whose  strength 
was  in  every  case  carefully  tested.  The  results  of  these  experi- 
ments are  given  in  the  following  tables. 


1912] 


Lipman-Sharp :   Phenoldisulphonic  Acid  Method 


25 


TABLE   I 

Effects  of  NaCl 

NaCl  added 

N.  added  as 

N.  found  as 

mgs. 

nitrate  mgs. 

nitrate  mgs. 

Uniform  quantities 

.25 

.050 

.045 

KNO  — varying 

.50 

.050 

.041 

amounts  NaCl 

1.00 

.050 

.035 

2.50 

.050 

.026 

Varying  quantities 

1.00 

.050 

.038 

K  Nonuniform 

1.00 

.100 

.070 

amounts    NaCl 

1.00 

.250 

.215 

1.00 

.500 

.460 

1.00 

1.000 

.940 

1.00 

2.500 

2.300 

Varying  quantities  of 

.25 

.050 

.046 

both  KNO    and  NaCl 

.50 

.100 

.078 

1.00 

.250 

.230 

1.50 

.500 

.460 

2.00 

1.000 

.900 

2.50 

2.500 

2.300 

Uniform  amounts  KNO 

.01 

.050 

.051 

small  amounts  NaCl 

.05 

.050 

.051 

.10 

.050 

.049 

Color  blanks 

.00 

.050 

.050 

on  both  salts 

2.50 

.000 

.000 

TABLE  II 

Effects  of  Na2S04 


Amounts  of  nitrate 
uniform  and  sulfate 
varying 


Amounts  of  sulfate 
uniform  and  nitrate 
varying 


Amounts  of  both  salts 
varying 


Na2S04  added 

N.  added  as 

N.  found  as 

mgs. 

nitrate  mgs. 

nitrate  mgs. 

1.000 

.0500 

.0480 

5.000 

.0500 

.0420 

10.000 

.0500 

.0400 

20.000 

.0500 

.0280 

30.000 

.0500 

.0270 

15.000 

.1500 

.1420 

15.000 

.5000 

.4950 

15.000 

1.0000 

.9000 

15.000 

2.0000 

1.9500 

15.000 

3.0000 

2.8500 

1.000 

.1500 

.1420 

5.000 

.5000 

.4800 

10.000 

1.0000 

.9200 

20.000 

2.0000 

1.9300 

26 


Un  iversity  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


TABLE 

III 

Effects  of 

Na2CO; 

i 

Na2C03  added 

N.  added  as 

N.  found  as 

mgs. 

nitrate  mgs. 

nitrate  mgs. 

Amounts  of  nitrate 

1.00 

.1000 

.0995 

uniform  and  carbonate 

2.50 

.1000 

.1040 

varying 

5.00 

.1000 

.1010 

10.00 

.1000 

.1010 

20.00 

.1000 

.1020 

30.00 

.1000 

.1020 

Amounts  of  carbonate 

10.00 

.1000 

.1000 

uniform  and  nitrate 

10.00 

.2000 

.2100 

varying 

10.00 

.5000 

.5000 

10.00 

1.5000 

1.4900 

Amounts  of  both  salts 

1.00 

.1000 

.1010 

varying 

5.00 

.2000 

.1970 

10.00 

.5000 

.5050 

20.00 

.5000 

.5100 

30.00 

1.5000 

1.4900 

The  results  set  forth  in  tables  I,  II,  and  III  leave  no  room 
for  doubt  as  to  the  effects  of  "alkali"  salts  on  the  nitrate  deter- 
mination by  the  colorimetric  method.  Both  NaCl  and  Na2S04 
induce  large  losses  of  nitrate,  and  especially  is  this  true  of  NaCl, 
which  may  be  responsible  for  losses  equivalent  to  forty-five  per 
cent  and  more  of  the  total  nitrate  present  as  indicated  in  Table 
I.  While  Na.,S04  induces  smaller  absolute  losses  than  NaCl,  they 
are  none  the  less  marked,  and  where  large  amounts  of  the  sulfate 
are  present  very  considerable  losses  of  nitrate  occur. 

Perhaps  the  most  striking  feature  of  the  foregoing  results  is 
what  appeals  to  one  at  first  sight  as  the  singular  difference  in 
the  behavior  of  Na2S04  and  Na2C03.  Whereas  the  former  is 
always  responsible  for  losses  in  the  determination  of  nitrates, 
the  latter  is  the  only  one  of  the  salts  tested  which  has  no  effect 
and  the  presence  of  which  in  a  long  series  of  tests  has  never, 
except  in  one  case,  decreased  the  amount  of  nitrate  present  as 
shown  by  the  colorimeter  readings.  It  was  naturally  assumed 
that  Na.CO.,,  after  the  addition  of  the  phenoldisulphonic  acid, 
would  be  converted  in  the  presence  of  an  excess  of  sulphuric 
acid  into  Na2S04  and  should  therefore  show  the  same  decreases 
in  the  nitrate  content  as  the  latter  salt.  To  clear  up  these  rather 
puzzling  facts,  as  above  given,  we  decided  to  run  a  special  series 
of  experiments  based  on   a  suspicion  which  we  had  as  to  the 


1912]  Lip  man- Sharp :   Phenoldisulphonic  Acid  Method  27 

nature  of  the  action  of  the  salts  in  question.  The  results  of 
these  experiments,  which  will  be  given  below,  make  entirely  clear 
what  seemed  at  first  quite  puzzling. 

In  further  general  discussion  of  the  tables  above  given,  it 
must  be  added  that  the  decreases  in  the  nitrate  content  of  the 
solutions  tested  as  induced  by  the  presence  of  salts  never  oc- 
curred in  accordance  with  any  definite  law,  the  losses  at  times 
being  greater  with  smaller  amounts  of  salts  than  with  larger 
amounts,  the  amounts  of  nitrates  being  constant.  On  the  other 
hand,  with  a  given  amount  of  nitrates  not  exceeding  one-tenth 
of  a  milligram  the  salts  seemed  always  to  induce  larger  per- 
centage losses  than  they  did  in  the  case  of  the  larger  amounts  of 
nitrates.  Our  results  not  only  give  good  opportunity  for  a  com- 
parison of  the  effects  of  varying  quantities  of  salts  on  the  same 
nitrate  content,  but  point  out  all  the  relationships  between  the 
salts  and  nitrates  where  first  the  former,  then  the  latter,  and 
finally  both,  are  varied.  There  are  two  other  points,  also,  which 
they  would  not  seem  to  confirm ;  indeed  they  give  entirely  different 
evidence  on  these  than  was  obtained  by  other  investigators.  The 
first  is  that  small  amounts  of  NaCl  do  not  induce  losses  of 
nitrates,  as  claimed  by  Stewart  and  Greaves,  and  Table  I  indi- 
cates that  amounts  of  NaCl  below  .1  milligram  do  not  occasion 
any  losses.  The  other  point  of  difference  between  our  results  and 
those  of  the  others  mentioned  is  that  Na2C03  does  not  decrease  the 
amounts  of  nitrates,  no  matter  to  what  extent  it  is  used,  as  shown 
in  Table  III.  This  is  in  entire  disagreement  with  the  results  of 
Gill  and  Chamot  and  his  coworkers,  who  claimed  that  Na2CO:} 
and  other  carbonates  induced  losses  of  nitrates,  in  the  determina- 
tion outlined.  It  must  also  be  added  here  that  the  effects  of 
Na2S04  as  given  in  Table  II  constitute  the  first  published  results, 
so  far  as  we  are  aware,  on  the  effects  of  Glauber  salt  on  the 
nitrate  determination,  and  they  have  indeed  been  indirectly  re- 
sponsible for  the  discovery  of  one  or  two  other  points  of  interest 
which  will  be  discussed  below. 

The  results  above  given  indicate  the  effects  of  each  of  the 
salts  taken  singly  on  the  nitrate  determination.  To  make  the 
data  more  complete  it  was  thought  desirable  to  test  various  mix- 
tures of  the  same  salts  and  note  their  effects.  Table  IV  gives 
the  results  obtained. 


28  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


TABLE 

IV 

Effects 

of  Mixed  Alkali  Salts 

ra2co3 

Na2S04 

NaCl 

N.  added  as 

N.  found  as 

mgs. 

mgs. 

mgs. 

nitrate  mgs. 

nitrate  mgs. 

1 

1 

1 

.1000 

.055 

5 

5 

5 

.1000 

.026 

10 

10 

10 

.1000 

.021 

20 

20 

20 

.1000 

.017 

1 

1 

.1000 

.082 

5 

5 

.1000 

.081 

10 

10 

.1000 

.075 

20 

20 

.1000 

.077 

1 

1 

.1000 

.061 

5 

5 

.1000 

.060 

10 

10 

.1000 

.055 

20 

20 

.1000 

.028 

1 

1 

.1000 

.050 

5 

5 

.1000 

.042 

10 

10 

.1000 

.033 

20 

20 

.1000 

.030 

10 

10 

10 

.2000 

.086 

10 

10 

10 

.5000 

.125 

10 

10 

10 

1.000 

.360 

10 

10 

10 

2.000 

1.140 

The  same  marked  losses  in  nitrates  occur  here  as  where  the 
salts  are  employed  singly.  NaCl  seems  to  be  responsible  again 
for  the  greatest  losses,  Na2S04  is  next  in  order,  and  Na2C03 
seems  to  have  little  or  no  effect.  Since  these  salts  occur  together 
in  alkali  soils,  however,  the  results  in  Table  IV  possess  consider- 
able significance  and  interest,  especially  since  they  point  out  what 
enormous  losses  of  nitrates  occur  where  such  large  amounts  as 
ten  milligrams  of  each  of  the  salts  are  added  to  the  nitrate-con- 
taining solution. 

The  Interference  of  Precipitants  of  Clay  and  Organic 
Matter  on  the  Nitrate  Determination 

It  is  very  singular  that  analytical  chemists  have  for  so  long 
a  time  been  employing  such  materials  as  saturated  alum  solu- 
tions, aluminum  cream,  and  bone  black  for  precipitating  clay 
and  organic  matter  in  obtaining  the  soil  solution  to  be  used  for 
nitrate  determinations  without  ever  having  attempted  to  ascer- 


1912] 


Lipman-Sharp :   Phenoldisulpho?iic  Acid  Method 


29 


tain  if  such  materials  in  any  way  affect  the  accuracy  of  the 
determination.  Indeed  Chamot  and  his  coworkers  have  recom- 
mended the  use  of  aluminum  cream  for  removing  suspended 
material  from  the  solution,  and  claim  to  have  had  very  satis- 
factory results  in  the  use  of  that  material.  Our  experiments 
in  this  series  were  intended  to  clear  up  this  question  and  the 
following  results  show  very  strikingly  that  none  of  the  materials 
mentioned  may  be  employed  in  the  nitrate  determinations  with- 
out incurring  very  serious  losses.  Table  V  gives  results  obtained 
in  the  use  of  potash  alum,  and  Table  VI  gives  results  obtained 
in  the  use  of  bone  black  and  aluminum  cream. 


TABLE  V 

Effects  of  K2A12(S04)4 

K2A12(S04)4 

N.  added  as 

N.  found  as 

added 

nitrate 

nitrate 

mgs. 

mgs. 

mgs. 

Amounts  of  nitrate 

5.00 

.050 

.040 

uniform  and  alum 

12.50 

.050 

.036 

varying 

25.00 

.050 

.033 

50.00 

.050 

.031 

100.00 

.050 

.034 

150.00 

.050 

.040 

Amounts  of  alum  uni 

form 

45.00 

.050 

.035 

and  nitrate   varying 

45.00 

.100 

.075 

45.00 

.250 

.168 

45.00 

.500 

.345 

45.00 

1.000 

.675 

45.00 

2.500 

1.800 

Amounts   of  both 

5.00 

.050 

.040 

salts  varying 

12.50 

.100 

.074 

25.00 

.250 

.175 

50.00 

.500 

.335 

100.00 

1.000 

.690 

150.00 

2.500 

1.850 

Color  blanks  on 

.00 

.050 

.049 

both  salts 

.00 
100.00 

.500 

.480 

30 


University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


TABLE  VI 
Effects  of  Aluminum  Cream  and  Bone  Black 


Sufficient  aluminum  cream  to 
clear  solution.     Five  minutes 
exposure 

Twice  the  amount  of  aluminum 
cream  used  above.  Exposed  one 
and   one-half  hours 


r.  added  as 

N.  found 

nitrate 

nitrate 

mgs. 

mgs. 

.5000 

.254 

1.000 

.648 

2.000 

1.460 

.5000 

.100 

1.0000 

.300 

2.0000 

1.180 

1.0000 

.135 

2.5000 

.650 

5.0000 

2.200 

Sufficient  bone  black  to  clear 
and  decolorize  solution 


The  data  in  Tables  V  and  VI  are  clearly  very  striking.  The 
enormous  losses  of  nitrates  sustained  through  the  use  of  a  satur- 
ated solution  of  alum,  varying  quantities  of  aluminum  cream  and 
bone  black,  make  these  substances  entirely  unfit  for  use  as 
precipitants  for  clay,  or  organic  matter,  or  both,  when  nitrates 
are  to  be  determined.  While  bone  black  occasions  the  largest 
losses,  and  potash  alum  the  smallest,  of  any  of  the  substances 
above  described,  the  losses  of  nitrates  brought  about  through  the 
use  of  all  the  precipitants  are  too  great  to  permit  of  their  con- 
tinuance in  a  method  for  nitrate  determinations  which  is  none 
too  accurate  under  the  best  of  conditions.  It  is  therefore  evident 
that  nitrates  are  lost  not  merely  through  the  loss  of  nitric  acid, 
as  is  the  case  where  salts  are  used,  but  that  there  is  a  loss  of 
nitrates  mechanically  through  adsorption  on  the  part  of  the 
colloidal  material  of  the  precipitant,  as  must  be  the  case  where 
such  substances  as  aluminum  cream  and  bone  black  are  used. 
The  large  amounts  of  colloids  possessed  by  these  substances,  with 
the  accompanying  large  surface  areas,  evidently  prevent  some  of 
the  nitrate  in  solution  from  going  through  the  filter. 

On  casting  about  for  a  method  to  precipitate  clay  or  organic 
matter,  we  first  tried  the  Briggs  filter  pump,  but  found  that  open 
to  two  objections.  First,  the  losses  of  nitrates  through  what  we 
look  upon  as  adsorption  on  the  part  of  the  clay  filter,  though  not 
very  large,  were  nearly  equal  to  those  induced  by  small  amounts 
of  sulfates.  Second,  while  the  filter  pump  yields  a  clear  solu- 
tion, it  docs  not  serve  to  decolorize  solutions.     After  several  fur- 


1912]  Lipman- Sharp :   Phenoldisul phonic  Acid  Method  .'>1 

ther  attempts  to  find  a  coagulating  and  decolorizing  agent  which 
might  promise  well  for  this  method,  it  struck  us  that  quicklime, 
being  the  best  coagulating  material  for  clay,  might  perhaps  also 
serve  to  remove  organic  matter  and  yet  might  not  decrease 
seriously  the  amount  of  nitrates  in  the  solution  to  be  tested. 
Accordingly,  tests  were  carried  out  by  adding  lime  to  solutions 
containing  known  amounts  of  nitrates,  to  soils  containing  known 
amounts  of  nitrates  and  to  soils  with  unknown  amounts  of 
nitrates,  in  which  latter  a  comparison  was  also  especially  made 
between  lime  and  aluminum  cream.  We  found  in  these  experi- 
ments that  the  losses  of  nitrate  through  the  use  of  lime  were  not 
only  very  small  or  negligible,  but  that  the  action  of  lime  in 
precipitating  both  clay  and  organic  matter  was  equal  to  or  better 
than  that  of  the  best  of  the  coagulating  and  decolorizing  agents. 
Its  coagulating  action  on  clay  has  of  course  always  been  recog- 
nized in  soil  physics.  The  results  of  the  experiments  are  given 
in  Table  VII. 

TABLE  VII 

Effects  of  Lime 
A — Solutions  of  known  nitrate  content 


CaO  present 

N. 

added  as  nitrate 

N. 

found  as  nitrate 

grms. 
1 

mgs. 
1.0000 

mgs. 
1.0150 

3 

1.0000 

.9800 

5 

1.0000 

.9550 

3 

5.0000 

4.6500 

B — Soils  of  known  nitrate  content 

CaO  present  N.  present  as  nitrate  N.  found  as  nitrate 
grms.                            mgs.  mgs. 

Lime  ground  with 

soil  and  water  2  3.280  3.150 

Lime  added  to  muddy 
suspension  2  3.280  3.200 

C — Comparison  of  lime  and  aluminum  cream  on  soil  of  unknown  nitrate 

content 


CaO  present 

N.  fou 

nd  as  nitrate 

grms. 

mgs. 

Lime  ground  with 

soil  and  water 

2 

1.210 

Lime  added  to 

muddy  suspension 

2 

1.225 

Sufficient  aluminum 

cream  added  to 

clear  solution 

„ 

.800 

32  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 

It  would  seem  from  these  results  therefore  that  lime  can 
yield  a  clear,  colorless  solution  without  decreasing  the  quantity 
of  nitrates  present  in  the  solution  appreciably,  and  that  it  is 
therefore  the  only  one  of  the  coagulating  agents  above  tested 
which  can  be  safely  used  in  the  work.  We  commend  it  to  soil 
chemists  and  others  making  nitrogen  determination  under  similar 
conditions.  Only  where  very  large  quantities  of  lime  are  em- 
ployed, and  they  are  not  necessary,  have  we  found  definite  losses 
of  nitrates.  We  find  that  2  grams  of  CaO  is  sufficient  to 
coagulate  the  clay  in  100  grams  of  loam  soil  and  to  remove 
whatever  color  may  be  present  at  the  same  time. 

While  lime  has  been  used  by  some  chemists  in  accordance 
with  the  method  above  outlined,  its  use  has  by  no  means  been 
general  and  no  data  prior  to  this  existed  with  reference  to  its 
effects  on  the  nitrate  determination.  J.  G.  Lipman  and  P.  E. 
Brown  give  directions  in  their  laboratory  manual  on  Soil  Bac- 
teriology for  the  use  of  2  grams  of  lime  to  precipitate  the  clay 
in  the  100  gram  samples  of  soil  used  in  nitrification  experiments, 
but  we  have  never  seen  any  published  statements  beyond  that 
as  to  the  advisability  or  feasibility  of  employing  lime.  It  is  cer- 
tainly surprising  that  those  who  have  tested  the  method  for 
nitrate  determination  should  not  have  tried  and  urged  the  use 
of  lime  as  a  substitute  for  alum  or  aluminum  cream. 

Other  Experiments  on  Salt  Effects 

It  appeared  interesting,  when  the  results  in  Tables  I,  II,  and 
III  were  obtained,  to  ascertain  if  the  kation  as  well  as  the  anion 
of  salts  was  responsible  for  losses  of  nitrates.  Accordingly  a 
series  of  experiments  was  instituted  in  which  the  effects  of 
NaCl,  KC1,  and  MgCL  could  be  compared.  The  following  re- 
sults were  obtained. 


1912] 


Lipman-Sharp :   Phenoldisulpl tonic  Acid  Method 


33 


KCl 
mgs. 

1 

5 
10 
20 


TABLE  VIII 

Effects  of  Ioxs 

X.  added  as 

X.  found  as 

MgCl2 

NaCl                       nitrate 

nitrate 

mgs. 

mgs.                           mgs. 

mgs. 

.... 

.1000 

.070 

.1000 

.063 

.1000 

.055 

.1000 

.050 

1 

.1000 

.057 

5 

.1000 

.028 

10 

.1000 

.016 

20 

.1000 

.011 

.... 

1                      .1000 

.065 

5                     .1000 

.043 

10                      .1000 

.035 

20                     .1000 

.038 

It  is  evident  from  Table  VIII  that  the  chlorine  and  not  the 
base  is  the  interfering  element,  and  while  the  amounts  of  chlorine 
were  not  so  proportioned  as  to  be  equivalent  in  the  case  of  the 
two  monovalent  bases,  the  effect  is  clearly  seen  of  the  smallest 
and  the  largest  amounts  of  chlorine  present  in  the  salts,  which 
can  be  calculated  from  the  molecular  weights.  The  negative 
ion  therefore  seems  to  be  the  active  agent  in  setting  free  nitric 
acid,  but  the  decreases,  depending  as  they  do  on  other  conditions 
such  as  evaporation  on  the  water  bath  and  length  of  exposure,  do 
not  take  place  in  accordance  with  any  definite  law. 

The  last  phase  of  the  salt  effects  studied  was  that  above  re- 
ferred to  in  the  discussion  of  Tables  I,  II,  and  III,  namely,  the 
reason  for  differences  in  the  action  of  Na2C03  and  Na2S04  on 
nitrate-containing  material.  Since  it  was  evident  that  Na2COo 
should  react  similarly  to  Na2S04  when  the  phenoldisulphonic 
acid  was  added  to  the  dried  residue  to  be  analyzed,  we  suspected 
that  the  losses  occurring  when  Na2S04  was  employed  came  about 
on  the  water  bath  in  evaporating  the  solution,  under  which  con- 
ditions only,  according  to  our  work,  could  there  have  been  a 
difference  in  the  action  of  the  two  salts. 

The  results  given  in  the  following  table  prove  that  our  sus- 
picions were  well  founded.  In  this  series  the  dry  salts  were 
thoroughly  mixed  with  the  nitrate-containing  residue  obtained  by 
evaporating  standard  nitrate  solutions,  and  then  the  phenoldi- 
sulphonic acid  reagent  was  added.    NaCl  was  similarly  tested. 


34  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 


TABLE  IX 

Effects  of 

Dry  Mixing  of  Nitrates  and 

Salts 

N.  added  as 

N. 

found  as 

a2C03 

Na2S04 

NaCl                       nitrate 

nitrate 

mgs. 

mgs. 

mgs.                         mgs. 

mgs. 

50 

.1000 

.097 

100 

.1000 

.102 

50 

.1000 

.103 

100 

.1000 

.102 

.... 

50                      .1000 
100                      .1000 

.080 
.062 

The  data  in  Table  IX  make  it  quite  clear  that  the  losses  due 
to  Na2S04  occur  only  when  the  latter  salt  is  present  in  solution 
with  nitrates  and  the  solution  is  evaporated  on  the  steam  bath. 
When,  however,  the  salt  is  mixed  dry  with  the  dry  nitrate  no 
losses  of  the  latter  occur  any  more  than  they  do  when  Na2C03  is 
added.  The  same  is  not  true,  however,  of  NaCl,  as  is  shown  in 
the  last  table.  That  salt  causes  losses  of  nitrates  during  both 
the  evaporation  on  the  steam  bath  and  the  reaction  setting- 
chlorine  free  in  the  treatment  of  the  dry  residue  with  phenoldi- 
sulphonic  acid.  This  latter  fact  is  a  confirmation  of  work  done 
by  Gill  and  reviewed  above.  We  have  thus  shown  the  individual 
reaction  of  each  of  the  salts  as  related  to  the  nitrate  determina- 
tion and  the  causes  which  are  responsible  for  the  difference. 
Nitric  acid  is  evidently  set  free  from  nitrates  through  the  com- 
bined action  of  heat  and  the  S04  radicle  on  the  steam  bath  and 
in  the  evolution  of  chlorine  when  the  phenoldisulphonic  acid  is 
added  to  nitrate  and  chloride-containing  material.  Na2C03, 
however,  possessing  only  a  weak  and  unstable  acid  radicle  is 
powerless  to  set  free  nitric  acid  either  through  the  help  of  heat 
on  the  steam  bath  or  by  its  reaction  with  the  phenoldisulphonic 
acid. 

General,  Remarks 

So  many  factors  may  interfere  with  the  determination  of 
nitrates  by  the  phenoldisulphonic  acid  method  that  it  would  ap- 
pear to  be  almost  worthless,  and  yet  it  would  seem  to  us  that 
since  there  is  no  other  good  method  to  take  its  place  which  is 
nearly  as  simple  and  capable  of  use  in  very  numerous  deter- 
iii i tuitions,  it  is  worth  while  taking  certain  precautions  to  avoid 


1912]  Lipman-Sharp :   Phenoldisulphonic  Acid  Method  35 

error,  and  to  establish  the  method  on  a  firmer  basis.  Our  results 
as  above  outlined  show  that  losses  of  nitrates  are  induced  by 
the  presence  of  NaCl  and  Na2S04,  and  such  losses  are  indeed 
hard  to  avoid  when  working  with  "alkali  soils."  Even  the 
suggestion  of  Chamot  that  AgS04  might  be  used  to  precipitate 
chlorides  would  seem,  from  our  results,  not  to  be  useful,  since 
the  addition  of  sulfate  to  the  solution  would  accomplish  very  con- 
siderable losses  itself,  even  if  the  silver  sulfate  can  be  obtained 
nitrate-free,  which  Chamot  claims  is  seldom  the  case.  So  that  while 
we  deem  it  unsafe  in  the  presence  of  considerable  quantities  of  salts 
containing  chlorides  and  sulfates  to  determine  nitrates  by  the 
phenoldisulphonic  acid  method  and  would  therefore  recommend 
the  Street  modification  of  the  Ulsch  method  in  such  cases,  it  is 
likewise  clear  that  many  of  the  nitrate  determinations  made 
in  soil  laboratories,  as  is  especially  the  case  in  soil  bacteriological 
work,  would  not  be  interfered  with  by  salts.  In  such  cases  the 
method  can  be  safely  depended  on  if  potash  alum,  aluminum 
cream,  and  bone  black  are  not  used  to  coagulate  clay  and  or- 
ganic matter,  since  they  have  been  found  in  the  researches  above 
described  to  be  productive  of  very  serious  errors.  We  recom- 
mend as  a  substitute  for  these  coagulating  agents  the  oxide  of 
lime  in  its  chemically  pure  state,  to  be  employed  in  accordance 
with  the  method  above  given.  The  losses  of  nitrates  sustained 
through  its  use  have  been  shown  to  be  very  small  in  the  work 
above  reported,  and  it  may  be  employed  by  grinding  the  soil 
with  water  or  by  direct  addition  to  the  muddy  suspension  pre- 
pared from  the  soil. 

Other  sources  of  loss  such  as  those  brought  about  through 
the  sterilization  of  controls  in  the  autoclave  are  unavoidable. 
They  have  been  found  at  times  to  be  distinctly  appreciable,  and 
especially  in  the  presence  of  considerable  quantities  of  organic 
matter.  It  is  further  of  the  greatest  interest  to  learn,  from  the 
experiments  above  described,  of  the  action  of  the  anion  of  the 
salts  employed  in  our  studies  and  the  losses  of  nitrates  occurring 
on  the  water  bath  from  solutions  being  evaporated  there  when 
either  NaCl  or  Na2S04  is  present. 

We  should  also  make  mention  here  of  our  attitude  toward 
the  use  of  NH4OH  instead  of  KOH,  which  was  found  superior 


36  University  of  California  Publications  in  Agricultural  Sciences    [Vol.  1 

to  the  former  in  the  investigations  above  reviewed.  While  higher 
absolute  results  may  no  doubt  be  obtained  from  the  use  of  KOH 
than  from  NH4OH,  and  while  in  addition  ammonia  possesses 
other  objectionable  features,  we  were  not  aware  of  the  first  of 
these  objections  when  these  investigations  were  begun  and  did 
not  deem  the  others  serious  enough  to  warrant  a  change  in  the 
established  method.  Moreover,  the  same  relative  values  would 
exist  for  the  data  above  given  if  obtained  with  one  or  the  other 
of  the  hydrates,  and  therefore  our  results,  having  been  obtained 
throughout  by  the  use  of  ammonia,  do  not  in  any  way  lose  their 
value.  We  do  intend,  however,  in  the  future  to  employ  KOH 
exclusively  in  nitrate  determinations  made  in  this  laboratory. 
Finally  we  desire  to  call  the  attention  of  soil  chemists  to  the 
fact  that  losses  of  nitrates  by  the  agencies  above  described  never 
seem  to  occur  in  accordance  with  any  definite  law,  with  the 
exception  of  the  case  in  which  the  various  alkali  chlorides  are 
compared.  In  these  it  would  appear,  from  calculations  which 
we  have  made,  that  the  losses  of  nitrates  are  proportional  to 
the  amounts  of  chlorine  present.  While  no  law  can  be  formu- 
lated, however,  in  accordance  with  which  nitrates  are  lost  in 
the  presence  of  salts,  it  may  be  possible  to  work  out  tables 
for  the  losses  of  nitrates  incurred  in  the  presence  of  varying 
quantities  of  chlorides  and  sulphates,  and  to  make  corrections, 
therefore,  in  samples  whose  composition  is  unknown  after  alkali 
determinations  are  made.  It  is  true,  however,  that  calculation 
has  shown  on  the  basis  of  data  in  Table  VIII  that  the  losses 
of  nitrates  induced  by  chlorides  alone  are  proportional  to  the 
amount  of  chlorine  present. 

CONCLUSIONS 

1.  The  "alkali"  salts  NaCl  and  Na2S04  induce  losses  of 
nitrates  when  the  latter  are  determined  by  the  phenoldisulphonic 
acid  method.  Na2C03  has  no  such  effect.  NaCl  induces  much 
greater  losses  than'Na2S04. 

2.  Among  the  substances  used  to  coagulate  clay  and  organic 
matter  from  solutions  in  which  nitrates  are  to  be  determined, 
potash  alum,  aluminum  cream,  and  bone  black  have  been  found 
decidedly  unreliable.    They  all  induce  large  losses  of  nitrates. 


1912]  lAjman— Sharp :   Phenoldisulphonic  Add  Method  37 

.'3.  Lime  has  been  found  to  be  much  more  reliable  for  the  pur- 
pose named  than  any  of  the  other  substances,  the  losses  incurred 
through  its  use  being  very  small. 

4.  The  reason  for  the  difference  between  the  action  of  Xa2S04 
and  Na2C03  so  far  as  the  nitrate  losses  are  concerned  is  to  be 
found  in  the  fact  that  Na2S04  induces  the  loss  of  nitric  acid 
from  the  solution  while  the  latter  is  being  evaporated,  while 
Na2C03  containing  only  a  weak  acid  radicle  has  no  power  to 
set  nitric  acid  free.  Neither  Na2S04  nor  Na2C03  has  the  power 
to  set  nitric  acid  free  from  nitrates  when  the  dry  residues  of  the 
two  are  mixed  prior  to  treatment  with  phenoldisulphonic  acid. 

5.  Losses  of  nitrates  from  solutions  as  induced  by  chlorides 
alone  seem  to  be  proportional  to  the  amount  of  chlorine  present. 

6.  The  work  of  Gill  which  showed  that  chlorine  induces  losses 
both  on  the  water  bath  and  in  mixing  the  dry  residue  with 
phenoldisulphonic  acid  is  confirmed. 


